Im currently using an All Test Pro 31 for motor testing. The result of my insulation resistance reading shows 15 Megohms and fall in yellow alarm in the condition calculator.

However, when using the FLUKE Meg-Ohmmeter instrument in testing the insulation resistance,the result shows 120 Megohms. Using these results, it indicates that the winding is still in good condition.

My questions are:

1. Why the result of insulation resistance has a big difference from the ALL TEST PRO 31 compare to FLUKE Meg-Ohmmeter?

2. Which of these test instruments (ALL TEST PRO 31 and FLUKE Meg-Ohmmeter) are more accurate in terms of insulation resistance testing?

Reggie:

A few quick questions:

1. Did you discharge the insulation system prior to using the Fluke?

2. Did you apply the instruments for the same length of time?

3. Were the instruments set for the same voltage.

The insulation resistance of an insulation system increases over time as the insulation system polarizes. The result is a curve that can be viewed if you take timed measurements and graph them. The insulation system contains the energy just as if you were stretching a rubber band. You eventually need to release some of that energy which is done by discharging it to ground.

What you are actually reading is a measurement of leakage across the insulation system in milli-, or micro-, Amps. That value is converted to insulation resistance.

Now, the AT31 will apply power only as long as you hold down the insulation resistance button, then will display the results. Something similar happens with the Fluke instrument.

The accurate method for testing the insulation system is to follow a standard, such as IEEE 43, in which you apply the appropriate voltage to the insulation system for one minute, or until the instrument reads its maximum. You then discharge the insulation to ground for 4 times the amount of time the potential (voltage) was applied.

The instrument voltages should also be set the same (500 or 1000 Volts).

You should then see the measurements look similar.

Sincerely,
Howard

quote:

Originally posted by Reggie:
Im currently using an All Test Pro 31 for motor testing. The result of my insulation resistance reading shows 15 Megohms and fall in yellow alarm in the condition calculator.

However, when using the FLUKE Meg-Ohmmeter instrument in testing the insulation resistance,the result shows 120 Megohms. Using these results, it indicates that the winding is still in good condition.

My questions are:

1. Why the result of insulation resistance has a big difference from the ALL TEST PRO 31 compare to FLUKE Meg-Ohmmeter?

2. Which of these test instruments (ALL TEST PRO 31 and FLUKE Meg-Ohmmeter) are more accurate in terms of insulation resistance testing?

All that Motor Doc said is right on. One other issue to be concerned with is the voltage drop. When a DC voltage is applied to a load with a battery operated tester the voltage at the motor could be substantially different compared to the read out on the meter. This voltage drop is due to configuration of the meter. The charge of the battery will also have an effect on the applied voltage to the motor. To view what the voltage drop is with your meter use a DVOM and measure the voltage at the connection point of the meter. You will notice that larger motors will provide a larger voltage drop.

Actually, I was still at ATPro when the engineers developed the AT31 and did much of the field testing. The problem occurs when the user just presses the insulation resistance button briefly instead of holding it for the allotted time. The voltage drop occurs, initially, for about 1-2 seconds before the electronic package compensates.

While my preference with the ATPro equipment (I use machines from several of the vendors nowadays, depending on what my clients have selected) is still the AT4, I did use the AT31 on machines over 10,000 hp and one 13.2kV, 9000 hp machine without any problem even with the 1kV insulation resistance test limit of the instrument.

The condition calculator will show red below 5MegOhms and Yellow from 5 to 100 MegOhms (I know, I wrote the software).

Howard

The condition calculator will show red below 5MegOhms and Yellow from 5 to 100 MegOhms (I know, I wrote the software).

Howard[/QUOTE]

Dr. Howard, correct me if im wrong but the condition calculator which i was using (from BJM corp.) will show red if below 5MegOhms and Yellow if below 20MegOhms.

A very big thanks to all who contributed brighter ideas.

Reggie,

Conerning insulation to ground resistance testing, please keep in mind that resistance to ground can be an ever changing figure, depending mainly on temperature and humidity swings.

It has been my experience that the most accurate way to measure resistance to ground is to use an instrument designed for that purpose and to conduct it over some set time measurement if you want to use it for historical purposes, such as a megohm or PI test, testing at least two times nameplate voltage.

I have been using insulation to ground testing equipment as a standard practice for many years, testing at 1000 volts thinking that I was doing a super bang up job of it, until the day I found 1000 volts wasn't enough to find the problem!

I think every type of VOM, meggar, including my motor tester at 1000 volts, did not find this problem! (This motor circuit was on the CBM list for an annual check) Only when I used the step voltage test, did the problem surface at 1500 volts! I would caution you here to use an instrument that shuts down automatically when the breakdown occurs so as to not damage your motor windings or punch a hole in the windings, rendering the motor inoperable.

Jim:

You are not comparing apples to apples.

The test that was discussed is a MegOhm test that follows IEEE 43, which is looking for direct leakage problems. If there are contamination or overheating issues with the insulation system, it allows for a correctible condition.

If you are using high voltage testing on existing machines in application, even if you allow for electronic shut down, and you have winding contamination, damage will occur (latent damage if the system shuts down fast enough or there is any gap in the insulation system). I am sorry, but electronic instruments do not operate at the speed of an electron.

This is supported by the EASA AR-100 standard which requires repair shops to clean and dry insulation systems prior to applying any high voltage test.

Howard

Howard,

Thanks for pointing out the apples versus oranges viewpoint. I'll agree with you here because the problem of which I was speaking was a wiring problem and not a motor winding problem. This is a case study that will be presented at PdM 2006 about a repetitive intermittent fuse blowing problem.

Reggie,

I would suggest that some sort of standard test block for each of your tester functions either be purchased or constructed to compare the readings of your different instruments. When doing a megohm test, stick a volt meter across the leads to see what the actual output voltage of the test instrument is for comparison. If the output voltages are similar, the test results should be about the same.

Keep your test block in an air conditioned space to minimize temperature and humidity fluctations.

Not only could this test block be used for comparing instruments, it could also be used for a monthly test to make sure your instrument is still performing to its original manufactures calibration.

Another use for this device would be in the instance where you questioned your test instrument results and wanted to verify that it is still functioning properly by comparing it to historical trended data.